Cathode ray tube having divergent deflection plates for converging three beams

ABSTRACT

In a cathode ray tube in which a plurality of electron beams are focused on a screen by a focusing lens from which at least certain of the beams emerge along paths divergent with respect to the tube axis, and each of the beams emerging along a divergent path is deflected to cause convergence of the beams at a common area of the screen; such deflection of each diverging beam is effected by a pair of plates at different electrical potentials disposed at opposite sides of the respective divergent path to electrostatically deflect the beam therefrom, and the plate which is at the side of the respective divergent path away from which the beam is deflected is also divergent from the tube axis in the direction toward the screen so as to avoid impingement on such plate of electrons from the respective beam.

oshida et al.

15 3,678,318 51 *July 18, 1972 [72] Inventors: Susumu Yoshida; Akio Ohgoshl, both of Tokyo; Senri Miyaoka, Kanagawa-ken; Yoshiharu Katagiri, Tokyo, all of Japan [63] Continuation-impart of Ser. No. 697,414, Jan. 12,

1968, Pat. No. 3,448,316.

[52] US. Cl. ..3l3/70 C, 313/78 [51] Int. Cl ..H01j 29/50, l-IOlj 31/20, l-lOlj 29/70 [58] Field ofS earch ..3l3/69 C, 70 C, 92 B 56] References Cited UNITED STATES PATENTS 2,083,203 6/1937 Schlesinger ..313/70CX 2,711,493 6/1955 Lawrence ..3l3/7OCX 2,921,228 1/1960 Farnsworth ...3l3/69 C X 3,448,316 6/1969 Yoshida et al. .313/69 C FOREIGN PATENTS OR APPLICATIONS 946,527 1/1964 Great Britain ..313/70 C Primary Examiner-Robert Sega] Attorney-Albert C. Johnston, Robert E. Isner, Lewis H. Eslinger and Alvin Sinderbrand [57] ABSTRACT In a cathode ray tube in which a plurality of electron beams are focused on a screen by a focusing lens from which at least certain of the beams emerge along paths divergent with respect to the tube axis, and each of the beams emerging along a divergent path is deflected to cause convergence of the beams at a common area of the screen; such deflection of each diverging beam is effected by a pair of plates at different electrical potentials disposed at opposite sides of the respective divergent path to electrostatically deflect the beam therefrom, and the plate which is at the side of the respective divergent path away from which the beam is deflected is also divergent from the tube axis in the direction toward the screen so as to avoid impingement on such plate of electrons from the respective beam.

3 Claims, 9 Drawing figures IN VE N TORS SUSUMU YOSHIDA AKIO OHGOSHI SENRI MIYAOKA YOSHIHARU KATAGIRI FIG. 3.

PATENTEDJULIIBIQYZ 3.678.318

sum 1 UF 2 FIG.

FIG. 2.

FIG. 4.

FIG. 5.

El INVENTORS SUSUMU YOSHIDA AKIO OHGOSHI SENRI MIYAOKA B Y YOSHIHARU KATAGIRI PATENTED JUL 1 8 I972 2mm 2 UP 2 FIG. 7

0:0 400 V 010500; 0124001/ $743k 15%, -200 foamy SUSUMU YOSHIDA AKIO OHGOSHI N VENTORS Fl G. 9

SENRI MIYAOKA YOSHIHARU KATAGIRI BY J14.

A TTORNf-JY DEFLECTION PLATES FOR CONVERGING THREE BEAMS This application is a continuation-in-part of our copcnding application Ser. No. 697,414, filed Jan. 12, 1968, and issuing June 3, 1968 as US. Pat. No. 3,448,316.

This invention generally relates to cathode ray tubes, and more particularly is directed to improvements in color cathode ray tubes of the type in which a single electron gun is provided for emitting a plurality of electron beams to produce a color picture, for example, as in color television receivers.

Existing color picture tubes are usually of the multigun type and include three independent electron guns emitting respective electron beams which are modulated by corresponding color signals and acted upon by a grid system so as to be focused on a collector or electron-receiving screen which may be simply a phosphor or luminescent screen or a phosphor screen with a perforated electrode or shadow mask in front thereof. The three electron guns have to be aligned with respect to each other so that the emitted electron beams converge at the electron-receiving screen. Such color picture tubes of the multi-gun type are disadvantageous in that it is difficult to obtain and maintain the precise alignment of the three electron guns required for the convergence of their beams on the electron-receiving screen and any misconvergence of the beams causes deterioration of the quality and resolution of the color picture that results. Further, when using three independent electron guns to produce the beams, the color picture tube is necessarily costly and, by reason of the space required for the three guns, the possible miniaturization of the tube is correspondingly limited.

In an attempt to avoid the above mentioned disadvantages and limitations of the existing color picture tubes of the multigun type, it has been proposed to provide a color picture tube of the single-gun, plural-beam type in which a single electron gun emits three beams from either three respective cathodes or a single cathode, and the three electron beams are passed through a lens-like focusing system, so as to converge at the electron-receiving screen. However, in the tubes of the singlegun, plural-beam type heretofore proposed, no more than one of the electron beams passes through the lens-like focusing system at the optical axis of the latter, and the beams that pass through the focusing system at a distance from the optical axis are subject to coma and spherical aberration. By reason of such coma and spherical aberration and the consequent deterioration of the quality of the color picture that results, color picture tubes of the single-gun, plural-beam type have not enjoyed any widespread use.

Accordingly, it is an object of this invention to provide a cathode ray tube of the single-gun, plural-beam type which is free of the above mentioned disadvantages characteristic of tubes of that type as previously proposed, and which is particularly suited to serve as a color picture tube for producing color pictures of high resolution and brightness.

A more specific object is to provide a single-gun, pluralbeam cathode ray tube in which focusing of the beams is effected substantially without imparting coma or spherical aberrations thereto, and the beams are converged at a common area on the screen in a manner to avoid shadows or ghosts by reason of such convergence.

In accordance with an aspect of this invention, a cathode ray tube adapted for use as the picture tube of a color television receiver is provided with a single electron gun including a cathode structure emitting electrons which are formed, as by a grid structure, into a plurality of electron beams, such beams are converged to intersect each other substantially at the optical center of a lens-like, electrostatic focusing means which is common to all the beams and focuses the beams on the electron-receiving screen, whereby the introduction of spherical aberration and/or coma is diminished, the beams emerging from the focusing means along paths that are divergent from the optical axis of the latter are each passed between a pair of spaced plates at different electrical potentials to electrostatically deflect the beam toward such optical axis for convergence of the beams at a common area of the electron-receiving screen, and the plate of each pair away from which the respective beam is deflected is arranged to diverge with respect to the optical axis in the direction toward the screen to avoid impingement on such plate of electrons from the respective beam.

The above, and other objects, features and advantages of this invention, will become apparent from the following 0 detailed description of illustrative embodiments which is to be read in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic view illustrating the optical equivalent or analogy of a three electron gun system, as in a conventional color cathode ray tube;

FIGS. 2 and 3 are similar diagrammatic views of the optical equivalent or analogy of a single-gun, plural-beam system, as previously proposed;

FIG. 4 is a diagrammatic view of the optical equivalent or analogy of still another single-gun, plural-beam system as previously proposed;

FIG. 5 is a similar diagrammatic view showing the optical equivalent of an electron gun of the type to which this invention relates;

FIG. 6 is a schematic, axial sectional view, in a horizontal plane, of an electron gun corresponding to the optical analogy of FIG. 5;

FIG. 7 is a schematic, axial sectional view of a chromatron type color cathode ray tube embodying the present invention;

FIG. 8 is an enlarged fragmentary sectional view of the convergence deflecting means of FIG. 7, and which shows the impingement of electrons upon plates thereof which is to be avoided according to this invention; and

FIG. 9 is a view similar to that of FIG. 8, but showing the arrangement of the convergence deflecting means according to this invention.

In order that the electron gun for a cathode ray tube of the type to which the present invention relates may be better understood, the principles and features of conventional electron guns employing the triple-gun system and the single-gun, triple-beam system, respectively, will first be described in detail with reference to FIGS. 1 to 4.

FIG. I shows the optical equivalent or analogy of a conventional system employing three independent electron guns Al, A2 and A3. In such system, there are provided three independent beam generating sources K1, K2 and K3 emitting three beams B1, B2 and B3, respectively, which are focused onto an electron-receiving or phosphor screen S through separate main lens systems L1, L2 and L3, respectively. With such an arrangement, however, the three independent electron guns A1, A2 and A3 which need to be accommodated in the neck portion of the tube envelope obviously restrict the extent to which the diameter of the neck portion can be reduced. Further, if the effective diameter of each electron gun is limited so as to permit the accommodation of the three guns in a neck portion of reasonable diameter, the outer portions of each beam necessarily pass through parts of the respective main lens system L1, L2 or L3 which are spaced substantially from the optical axis thereof whereby spherical aberration results with the consequence that each beam impinges on the screen S as a relatively large spot, as indicated at the righthand side of FIG. 1, and high resolution cannot be obtained. It will also be apparent that, in using three independent electron guns, it is inherent that difficulties will be encountered in obtaining and maintaining the precise alignment of the guns necessary for converging the beams BI, 82 and B3 at screen S.

FIG. 2 shows the optical equivalent of a conventional singlegun, triple-beam system in which the single electron gun A includes equivalent beam generating sources K1, K2 and K3 spaced from each other by the distances d and from which three beams B1, B2 and B3 are emitted in parallel to each other so as to pass through the common main lens system L and be converged by the latter on the screen S.

Whether the electron gun system of a color cathode ray tube is of the triple-gun type (FIG. 1) or of the single-gun, triple-beam type (FIG. 2), it is necessary that the three electron beams be converged with an angle of A between the center beam (B2 in the drawing) and each of the other beams so that the three beams cross or intersect each other at the position of a mask or grid provided in front of the phosphor or luminescent screen and are thus made to land or impinge on respective color dots or stripes which are adapted to produce different color light rays.

In order to meet the foregoing requirements with respect to the angle A 0 in the single-gun, triple-beam system, it is essential that the three beams B1, B2 and B3 be spaced apart from each other by the substantial distance d when they pass through the main lens L. Thus, beams B1 and B3 pass through portions of lens L which are spaced substantially from the axis of the lens L by the distance d, so that the beam spots on the screen S are deformed, as shown at the right-hand side of FIG. 2, due to coma as well as to spherical aberration. In the case shown in FIG. 2, the focusing of the beams is adjusted to achieve perfect convergence at the screen S. This inevitably decreases the focusing effect imparted to each beam. Thus, the beams are under-focused so that the resulting beam spots are enlarged, as is apparent at the right-hand side of FIG. 2. On the other hand, if the focusing voltage is adjusted to sharply focus beam B2 on screen S, this causes the beam spots B1, B2 and B3 on the screen S to be scattered, as shown on FIG. 3. Therefore, special means have to be provided to converge or super-impose the beam spots which are thus scattered. However, even in that case, the beam spots B1 and B3 are deformed due to coma, as shown on the right-hand side of FIG. 3.

In an attempt to satisfy the contradictory conditions of focusing the three beams on screen S and of converging the three beams at the screen, it is conceivable that the three beams B1, B2 and B3 could be emitted from a beam generating source K in three different or angularly displaced directions so as to be spaced apart from each other a distance d at the position of the main lens L, as illustrated on FIG. 4. Although the above two conditions can thus be simultaneously satisfied with only negligible spherical aberration, nevertheless the side beam spots BI and B3 are blurred due to the coma, as shown at the right-hand side of FIG. 4, since the side beams pass through the main lens L at positions spaced from the axis of the lens by the distance d.

It will be seen from the above that cathode ray tubes employing the single-gun, triple-beam system as previously devised or proposed fail to satisfactorily meet the three-beam spot focusing condition and the three-beam spot focusing condition and the three-beams spot converging condition and therefore have not been put to practical use as yet.

In the following detailed description of illustrative embodiments of a single-gun, plural-beam systems to which this invention relates, particular reference is made to the use thereof in color picture tubes, but it is to be understood that the described single-gun, plural-beam systems can be applied to any other cathode ray tubes in which plural electron beams are required.

In the system to which this invention relates, as illustrated by its optical equivalent or analogy on FIG. 5, a single electron gun A includes equivalent beam generating sources K1, K2 and K3 which are located on a straight line in a plane substantially perpendicular to the axis of the electron gun and spaced apart from each other by a distance do. These beam generating sources K1, K2 and K3 emit three electron beams B1, B2 and B3, respectively, which are refracted by means of a common auxiliary lens L so as to be converged substantially at the optical center of a main lens L. Thus, the three beams B1, B2 and B3 are made to cross each other substantially at the optical center of the main lens L and then emerge from the lens L in divergent directions. Subsequently, the beams B1 and B3 which diverge from the optical axis and from the beams B2 lying on such axis, are deflected toward the center beam B2 by means of convergence deflectors F1 and F2 provided between the electron-receiving screen S and the main lens L and spaced from the latter by a distance I, so that the three beam spots B1, B2 and B3 are converged to a common area on the screen.

With the arrangement of FIG. 5, therefore, very small beam spots can be obtained since all three beams B1, B2 and B3 pass through the center of main lens L, and thus the focused beam spots are prevented from being blurred due to the coma and spherical aberration. Consequently, a picture with a high resolution can be produced. Furthermore, utilization of the deflectors F1 and F2 advantageously facilitates the dynamic convergence correction with respect to the three beams.

FIG. 6 shows the optical equivalent of another cathode ray tube of a type to which this invention relates and in which a single electron gun A includes beam generating sources K1, K2 and K3 arranged on an arcuate surface having its center at the optical center of a main lens L, and being spaced from each other by the straight distance do. In this embodiment, the auxiliary lens L of FIG. 5 is omitted, as the arrangement of the sources K1, K2 and K3 on the described arcuatc surface causes the three beams B1, B2 and B3 to cross each other at the optical center of the main lens L, as in the embodiment shown in FIG. 5. Deflectors F1 and F2 are provided along the paths of the two beams B1 and B3 which cross each other within lens L and then follow diverging emergent paths, and such deflectors cause the beams B1 and B3 to converge with the beam B2 at a common area on the screen S. Thus, good resolution of the picture can be obtained in the same manner as described above in connection with FIG. 5.

Although the beam generating sources K1, K2 and K3 in FIGS. 5 and 6 are spaced apart from each other by a distance do or do along a straight line, it is possible to arrange these beam generating sources at the vertices of an equilateral ,triangle, in which case deflectors, as at F1 and F2, may be provided for each of the three beams or for only two of them.

A particular example of the structure of an electron gun A corresponding to the optical analogy of FIG. 5 will now be described with reference to FIG. 7 in which the electron beam generating sources are constituted by three electrically separated cathodes KR, KG and KB to which red, "green and blue" video signals are respectively supplied. The three cathodes are arranged with their electron emitting surfaces in a straight line so as to be aligned with similarly arranged apertures glR glG and glB in a plate-like first grid G1. A second cup-shaped grid G2 has an end plate disposed adjacent grid G1 and formed with three apertures g2R, g2G and g2B which are respectively aligned with apertures glR, glG and glB.

Arranged in order following the grid G2 in the direction away from control grid G1 are successive, open-ended, tubular grids or electrodes G3, G4 and G5. Electrode G3 includes relatively small diameter end portions 3 and 4 and a larger diameter intermediate portion 5, and is supported with its end portion 3 extending into cup-shaped grid G2 and spaced radially from side wall 2 of the latter. Electrode G4 includes end portions 6 and 7 of a diameter larger than that of end portions 3 and 4 of electrode G3 and an intermediate portion 8 of still larger diameter, and electrode G4 is mounted so that end portion 4 extends into, and is spaced radially inward from end portion 6. Electrode G5 includes an end portion 9 of a diameter smaller than that of end portion 7 and a relatively larger diameter portion 10, and electrode G5 is mounted so that its end portion 9 extends into, and is spaced radially inward from end portion 7 of electrode G4. The several electrodes G3, G4 and G5, grids G1, G2 and the cathodes KR, KG and KB are all assembled together in the above described relation by means of suitable supports (not shown) of insulating material.

In operating the electron gun of FIG. 7, appropriate voltages are applied to grids G1 and G2 and to electrodes G3, G4 and G5. For example, a voltage of 0 to 400V is applied to the grid G1, a voltage of O to 500V is applied to the grid G2, a voltage of 13 to 20KV is applied to the electrodes G3 and G5, and a voltage of 0 to 400V is applied to the electrode G4, with the voltage of the cathodes as the reference. Therefore, the voltage distributions with respect to the grids and electrodes G1 to G5, and their lengths and diameters are substantially identical with those of a unipotential-single beam type of electron gun which includes a first single grid member a second grid provided with a single aperture. With the applied voltage distribution described above, an electron lens field is established between grid G2 and the end 3 of electrode G3 which corresponds to the auxiliary lens L of FIG. 5, and an electron lens field corresponding to the main lens L of FlG. 5 is fomied at the axial center of electrode G4 by the electrodes G3, G4 and G5.

In order to cause convergence of the beams BB and Br which emerge from electrode G5 along divergent paths, the electron gun of FIG. 7 further has deflecting means F that includes shielding plates P and P provided in spaced opposing relationship to each other and extending axially from the free end of electrode G5. Deflecting means F further includes converging deflector plates Q and Q, which are shown to be planar and mounted in parallel, spaced opposing relation to the outer surfaces of shielding plates P and P, respectively. The plates P and P and the plates Q and Q are disposed so that the beams BB, BG and BR pass between the plates P and Q, between the plates P and P and between the plates P and Q, respectively. A voltage equal to that imparted to the electrode G5 is applied to the plates P and P, and a voltage lower than that applied to the plates P and P by 200 to 300V is applied to the plates Q and Q. Thus, deflecting voltage differences are applied between the plates P and Q and between the plates P and Q which respectively constitute the deflectors F1 and F2 of FIG. 5 and are adapted to impart the deflecting action to the beams BB and BR, respectively, as described above in connection with the beams B3 and B1 on FIG. 5.

Thus, the three beams BR, BG and BB emanating from the cathodes KR, KG and KB are made to pass through the apertures GlR, glG and glB of grid G1 and are modulated with three different signals applied between the respective cathodes ,and grid G1. The beams BR, BG and BB pass through the common auxiliary lens L which is formed mainly by the grid G2 and electrode G3 and cross each other substantially at the optical center of the main lens L which is constituted mainly by the electrodes G3, G4 and G5. Then, the beams BB, BG and BR pass between the plates Q and P, between the plates P and P and between the plates P anti Q respectively, after having left the electrode G5. Since plates P and P are at the same potential, beam BG is not deflected, but the beams BB and BR which emerge from lens L along divergent paths are deflected, so that the three beams are made to converge or cross each other at an aperture of a beam selecting aperture grill or shadow mask Gp and then diverge therefrom to impinge on a color screen S, comprised of sets of red," green and blue phosphor stripes or dots SR, SG and SB successively arranged on a face plate FP of the tube. The apertured grill or shadow mask Gp provided in front of color screen S may have a medium high voltage VM applied thereto. Voltages VP and VQ applied across the electrode plates P and Q and across the plates P and Q of convergence deflector means F are selected so that the three beams BR, BG and BB are made to cross each other at the position of the grill or mask Gp and thus made to land only on the corresponding phosphor stripes or dots SR, SG and SB. In this case, of course, the beams BR, BG and BB, while converging at the grill or mask Gp, are focused on the screen S.

The usual horizontal and vertical deflection means, as indicated by the yoke D, are provided for horizontally and vertically scanning the three beams simultaneously with respect to the screen S as in the conventional picture tube.

Thus, by supplying red, green and blue color video signals between the cathodes KR, KG and KB and the grid G1, respectively, the three beams BR, BG and BB are intensitymodulated, whereby a color picture is produced on the color screen.

Referring now to FIG. 8, it will be seen that, due to the potential difference of about 3003 between plates P and Q and between plates P and Q, beams BB and BR passing therebetween are, for the most part deflected inwardly, that is,

away from plates Q and Q, respectively, for convergence with central beam BG at an aperture of grill or mask Gp. However, particularly when the beam current is strong, the outer parts of the bundles of electrons making up beams BB and BR more or less continue to travel in the directions diverging from the tube axis. If the outer plates 0 and Q are planar and parallel to the tube axis, as shown in FIG. 8, or if such plates are bent so as to be outwardly convex, the electrons that are not fully deflected with the respective beams BB and Br, as at EB and Er, impinge against plates Q and Q. Such electrons impinging against the inner surfaces of plates Q and Q deteriorate the latter and further are reflected or deflected therefrom, as shown, to form beams of electrons reaching the screen S at locations spaced from the landing spots of the truly converged beams BB and BR. The beams of electrons reaching screen S after impinging on plates Q and Q may form shadows or ghosts.

In accordance with this invention, the undesirable impingement of electrons against outer plates Q and Q and the consequent deterioration of the plates and formation of ghosts" are avoided by providing deflecting means F FIG. 9) which is generally similar to the previously described deflecting means F, but in which the outer plates Q and Q diverge with respect to the tube axis in thedirection toward the screen S. Preferably, as shown, plates Q and Q according to this invention diverge with respect to the tube axis by an angle that is at least as large as the angle included between the tube axis and each of the paths divergent thereto in which the beams BB and BR travel upon emerging from the focusing lens L.

With the arrangement according to this invention as shown on FIG. 9, electrons EB and ER that are not fully deflected with the bulk of the respective beams BB and BR do not impinge on the diverging plates Q and Q. Thus, such electrons EB and ER escaping from the deflected beam bundle do not cause deterioration of the plates Q and Q and follow random diverging paths toward the screen S. The electrons traveling along random diverging paths strike the screen at widely dispersed positions and, therefore, do not create a shadow or ghost" image of the picture being produced. In fact, the effect of the dispersed electrons is practically negligible. Thus, a sharply defined picture of improved resolution is obtained.

In the foregoing an electron gun embodying this invention has been described as being applied specifically to a color picture tube in which a single gun is employed to produce three electron beams which are intensity modulated with the usual red, green and blue" color signals. However, it is obvious that an electron gun in accordance with this invention can be used in any other cathode ray tube requiring a plurality of beams which are to be focused at a common spot or at separated spots on an electron-receiving screen.

Although an illustrative embodiment of this invention has been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to that precise embodiment, and that various changes and modifications may be made therein by one skilled in the art without departing from the scope or spirit of the invention.

What is claimed is:

l. A cathode ray tube comprising beam producing means for producing three electron beams and including a first source for one of said beams which is located on the axis of the tube and two additional sources for the other two beams which are equally spaced from said first source at opposite sides of the latter on a straight line extending diametrically across said axis, a screen having arrays of different color phosphors and being positioned to have said beams impinge on respective phosphors of said arrays for exciting the same, means for causing said beams to intersect each other at a location on said axis of the tube between said beam producing means and said screen, focusing lens means for focusing said beams on said screen, said focusing lens means having an equivalent optical center and being positioned to dispose said optical center thereof on said axis substantially at said location when said beams intersect so that said one beam emerges from said focusing lens means along said axis and said other two beams emerge from said focusing lens means along paths that are divergent with respect to said axis, a pair of first plates at equal electrical potential disposed at opposite sides of said axis for the passage therebetween of said one beam upon emergence thereof from said focusing lens means, and second plates spaced outwardly from said first plates for the passage between said first and second plates of said other two beams, said second plates being at an electrical potential different from that of said first plates to electrostatically deflect the respective other beams in the direction toward said axis for causing convergence of said other beams with said one beam at a common area of said screen, and said second plates being divergent with respect to said axis in the direction toward said screen for avoiding impingement on said second plates ofelcctrons from said other beams.

2. A cathode ray tube according to claim 1, in which an apertured beam selecting member is disposed in a plane in front of said screen, and the difference between the electrical potentials of said first and second plates is selected to cause said other beams to cross said one beam at the plane of said beam selecting member.

3. A cathode ray tube according to claim 1, in which each of said second plates diverges with respect to said axis by an angle at least as large as the angle included between said axis and said paths that are divergent with respect to said axis. 

1. A cathode ray tube comprising beam producing means for producing three electron beams and including a first source for one of said beams which is located on the axis of the tube and two additional sources for the other two beams which are equally spaced from said first source at opposite sides of the latter on a straight line extending diametrically across said axis, a screen having arrays of different color phosphors and being positioned to have said beams impinge on respective phosphors of said arrays for exciting the same, means for causing said beams to intersect each other at a location on said axis of the tube between said beam producing means and said screen, focusing lens means for focusing said beams on said screen, said focusing lens means having an equivalent optical center and being positioned to dispose said optical center thereof on said axis substantially at said location when said beams intersect so that said one beam emerges from said focusing lens means along said axis and said other two beams emerge from said focusing lens means along paths that are divergent with respect to said axis, a pair of first plates at equal electrical potential disposed at opposite sides of said axis for the passage therebetween of said one beam upon emergence thereof from said focusing lens means, and second plates spaced outwardly from said first plates for the passage between said first and second plates of said other two beams, said second plates being at an electrical potential different from that of said first plates to electrostatically deflect the respective other beams in the direction toward said axis for causing convergence of said other beams with said one beam at a common area of said screen, and said second plates being divergent with respect to said axis in the direction toward said screen for avoiding impingement on said second plates of electrons from said other beams.
 2. A cathode ray tube according to claim 1, in which an apertured beam selecting member is disposed in a plane in front of said screen, and the difference between the electrical potentials of said first and second plates is selected to cause said other beams to cross said one beam at the plane of said beam selecting member.
 3. A cathode ray tube according to claim 1, in which each of said second plates diverges with respect to said axis by an angle at least as large as the angle included between said axis and said paths that are divergent with respect to said axis. 